Pipettes are standard laboratory apparatus used to transfer liquids from one vessel to another, or to a slide, or to other sample receiving medium. A pipette is basically a hollow tubular vessel open at both ends and the liquid to be transported is drawn into the tubular vessel by applying suction to the upper end. The liquid inside the vessel is forced out by the application of positive air pressure to the upper end of the vessel. Thus, in operation, liquid is first sucked into the vessel and then blown out of the vessel. For many years, the suction was applied to a pipette by mouth. This method has largely been abandoned because of health concerns. In modern pipette devices, the suction is applied by a flexible bulb or by a piston moving in a cylinder or a handheld vacuum gun that uses a small vacuum pump to suck the fluid into the pipette. Dispensing from the pipette is generally achieved by reversal of the action that created the suction.
The device of the present invention is designed to be used preferably in a pipette device in which the suction is provided by a spring-loaded piston moving up and down within a cylinder. The travel of the piston controls the volume of the fluid drawn into the pipette and dispensed from the pipette and the volume is adjustable by adjusting the piston travel.
In this manner, a precise volume of liquid can be drawn into the pipette and dispensed. A problem exists in that the suction device in pipette devices often become contaminated. One known source of contamination is overpipetting wherein the liquid being drawn into the pipette is sucked up into the suction device. It is also suspected that aerosols are generated from the liquid when the liquid is drawn into the pipette and these aerosols flow up into the suction device and contaminate it. Simple observation of the liquid in the pipette confirms this suspicion. When the liquid is moving through the pipette tip orifice, droplets of the liquid are caused to jump upwardly in an uncontrolled manner generating aerosols of the liquid. Also, aerosols can be generated when the liquid is dispensed from liquid left on the inner wall of the tip during the dispensing and this liquid is then subsequently sprayed up into the interior of the pipette and, ultimately, into the suction device when the suction device is allowed to return in a rapid manner to its home position drawing air into the tip of the pipette. In addition, fluid on the interior walls of the pipette tip during sequential pipettings forms thin films on the inner wall of the pipette tip. These films can migrate up the pipette tip inner wall surface above the column of liquid in the pipette and burst generating aerosols when the surface tension is insufficient to hold them on the inner wall of the tip. Pipetters typically use a pipette tip which attaches onto the barrel of a tube which contains a piston to apply suction to and expel a sample from the pipette tip. Thus, a contaminated pipette tip can be replaced with a new pipette tip for transporting a different liquid. However, the aerosols easily travel beyond the tip and up into the tube barrel contaminating this part of the system. In addition, if overpipetting takes place wherein the liquid sample is drawn up into the barrel of the suction device, catastrophic contamination occurs. Once the suction device is contaminated, new samples will pick up contamination from the previous samples even when a clean pipette tip is used. The carry over contamination can be a source of error in the on-going and subsequent assay procedures. In DNA applications wherein the DNA replicates in an exponential manner as well as in microbiological and radioactive pipetting procedures using hazardous fluid, cross-contamination from previous liquids cannot be tolerated. Moreover, if the barrel or suction device becomes contaminated, it may be impractical or impossible to clean and decontaminate it and thus the suction apparatus becomes unusable.
To avoid the aerosol problem, manufacturers of pipette devices have employed a porous media in the pipette tip to block aerosols of the pipetted liquid from reaching the suction device. The porous media allows the passage of air, but is intended to block the passage of liquid aerosols. Porous plastic is a particular suitable material for this application and several manufacturers are selling pipette tips with porous plastic plugs for the purposes of aerosol prevention. Porous plastic plugs are effective in blocking the passage of aerosols, but most of them do not prevent the flow of liquid into the suction device in the event of overpipetting.
To avoid this latter problem, a porous plastic plug has been developed which includes a self-sealing additive that seals off immediately when contacted by an aqueous liquid. Such a device is effective both in preventing aerosols and preventing contamination from overpipetting. However, the self-sealing additives to the porous plastic plugs contain sodium or other compounds which themselves are potential sample contaminants. For example, aerosols created on the initial aspiration of the liquid can come into contact with the porous plug and pick up sodium from the self-sealing additive and then fall back into the sample. In this manner, the sample would become seriously contaminated. Such sample contamination is often an expensive problem because some samples can cost thousands of dollars.
If overpipetting occurs, the liquid in the pipette tip comes into contact with the plug and causes the plug to seal itself closed. It then becomes impossible to extract the liquid from the pipette tip without cutting the pipette tip apart.
The self-sealing materials employed in the porous plastic plugs cannot be autoclaved to sterilize the pipette tips because the moisture induced by autoclaving will activate the self-sealing additive and cause the plug to seal itself off.